Airpreheater – Introduction, Working, Types, Applications

A very basic word to word meaning is a device used to heat the air before processing it to further use. They are also recognized as air heaters or air-heating pipe. It is designed to work to exchange heat energy with desuperheaters. Desuperheater is a Device which is been used to reduce the temperature of the steam in a high heat generation plants where large amount of heat energy or steam is released in the atmosphere.

What is Airpreheater?

A very basic word to word meaning is a device used to heat the air before processing it to further use. They are also recognized as air heaters or air-heating pipe. It is designed to work to exchange heat energy with desuperheaters. Desuperheater is a Device which is been used to reduce the temperature of the steam in a high heat generation plants where large amount of heat energy or steam is released in the atmosphere.

Working
Airpreheater can be performed by fashion counter flow, co-current flow, co-current flow-countercurrent or cross-countercurrent or cross-co-current flow. The design of the device is depend on the media and the temperature conditions of the application. For small construction, finned tubes are suited to arrange in a small space much heating surface. Is in the exhaust gas or in the air containing dust or other particles, so that deposits are expected, preferably smooth tubes are used, which are less prone to deposits and will be clean easier.
In most of the applications, an Airpreheater is performed in several stages that mean, it consists of several individual heat exchangers connected in series on the air side.
As they are heating the air passing through it by means of wasted heat energy airpreheateras are used in various areas such as boilers, industrial furnace units and in hot-air ventilation systems. They are designed depending on the applications where they are going to be used.
Types of Airpreheater
According to the working principle airpreheaters are divided into two types
• Recuperative or tubular type
• Regenerative or feed water type
Lets discuss one by one type in brief

1. Recuperative or Tubular Type
Recuperative type of airpreheaters is preferably used in small and medium size boilers. In this type heat energy exchange is carried out through the walls of heating surface that separate the heat energy carrier and air to be heated.
Ambient air is forced by a fan through ducting at one end of the preheater tube and at other end the heated air from inside the tubes emerges into another set of ducting Tubular preheaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. The hot furnace gases pass around the preheater tubes inside the ducting which transfers that carries it to the furnace of a boiler for combustion.
2. Regenerative or Feed Water Type
In feed water airpreheaters exchange of heat energy takes place through alternate heating and cooling of metallic or ceramic nozzles of the fixed or rotating heating surfaces. Regenerative air preheaters are compact and can have stationary rotating hood.
Types of Regenerative
Rotating-plate Regenerative

The rotating-plate design consists of a central rotating-plate element which is installed within a
bi-sector type- with Single Gas and Air Stream
tri-sector type- with Single Gas Stream but two air streams
or quad-sector type sectors which contain seals around that element.
Stationary-plate Regenerative

In this type the heating plate elements of regenerative air preheater are installed in a casing, but the heating plate elements should be stationary. Instead the air ducts in the preheater are rotated so as to alternatively parts of the heating plate elements to the up filling cool air.

Combine Use of Recuperative and Regenerative Type
The combination of tubular and regenerative type is used in the applications where very high capacity boilers are required. In such a type of combination the tubular type being used as a primary air heating and regenerative preheater is used for the secondary air heating purpose. If in case the designers of boiler dont want to use the combination of recuperative and regenerative type of Airpreheater, then they can go for tri-sector regenerative air heater.

Advantages of AirPreheater
It is used to heat the air before it is used for further application. For heating the air it uses heat energy of flue gases hence there are many more advantages of Airpreheaters over the normal preheaters such as
• Increases thermal efficiency of the system
• Payback period is very short and increase combustion rate
• Improve and strengthen the combustion
• Reduces the loss of furnace and the exhaust gas temperature, improve the boiler thermal efficiency.
• Airpreheater is suitable for all standard fuels
• They are easy for retrofit into existing systems
• These airpreheaters are suitable for all waste heat boiler / flue gas boiler / boiler types.
• They strengthen the heat transfer

Applications of Airpreheater
Because of the advantages discussed above Airpreheaters are used in various kind of applications from household devices to industrial purpose
• In refrigeration economizer system
• Ovens used in home appliances and industrial uses
• The dryer applications are heavy energy consumers in the food and non-food industry. Hot exhaust stream is transferred to the cold air inlet stream by using the Airpreheater.
• In the applications where indirect fired air heaters are required.

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